In certain applications it is desirable to store either ‘heat’ or ‘cold’ in thermal energy storage apparatus (hereinafter referred to as a “heat store”) containing thermal storage media, which media is able to store the thermal energy efficiently until it is subsequently retrieved. The ‘heat’ or ‘cold’ is transferred to and from the thermal storage media by a fluid, normally a gas (e.g. an inert gas), that is in direct contact with the thermal storage media. The thermal storage media should have a high surface area to facilitate energy transfer, good porosity for gas flow, and a suitable thermal heat capacity in the temperature range of operation of the store. Usually, the storage media is provided as a porous particulate or porous solid media contained within an insulated pressure vessel. Particularly where the store forms part of an energy storage system where large amounts of heat or cold need to be stored, the mass of thermal storage media required can be in 10's or 100's of metric tonnes, requiring the construction of large, expensive pressure vessels. In such vessels, therefore, all dead space must be kept to a minimum.
As mentioned above, a higher surface area leads to better energy transfer. If smaller particles (or channels or pores in solid media) are used, then the surface area tends to increase per unit volume of storage media—i.e. it is said to have a higher “specific surface”.
For example:—
                Packed bed spheres 10 mm diameter (cubic packing) approx 314 m2/m3         Packed bed spheres 1 mm diameter (cubic packing) approx 3140 m2/m3         Porous metal foam 5 pores per inch (12% density) approx 430 m2/m3         Porous metal foam 40 pores per inch (12% density) approx 2100 m2/m3 This shows that packed spheres with 1 mm particle size have a specific surface of approximately 3140 m2 of surface area in each cubic meter. For the porous foam metal with 40 pores per inch there is a specific surface of 2100 m2 of surface area in each cubic meter. The density of the foam metal is 12% of the solid, which means that it has a void fraction of 88%. The void fraction of the spheres in this example is approximately only 50% by way of comparison.        
There is a further advantage of using smaller particles with a higher specific surface. If smaller particles are used there are less “irreversible” thermal losses, since the particles equilibrate better and suffer less from internal thermal mixing (which would lead to a lower mean temperature for a particle after charging than the highest gas temperature experienced at its exterior, and hence, would lead upon discharge to the gas being reheated to a lower temperature). However, while these ‘irreversible’ thermal losses can be reduced by reducing the particle size, this increases gas pressure losses through the stores.
Particle size also affects store utilisation in terms of its effect on the thermal front. In a heat storage situation, a ‘thermal front’ is created in the storage vessel, i.e. a rise or a fall in temperature in the storage media and/or the gas with distance moved downstream, which occurs in an active region of the store i.e. where thermal transfer is most active.
FIG. 1 illustrates the formation of a thermal front in a thermal store and shows how the process of charging a thermal store sets up a thermal front within a region of the store that progresses downstream and that is usually initially quite steep but which (for a gas entering a store with storage media at a lower temperature) becomes progressively shallower as charging continues. Thus, the front starts with length L1, but as it moves down the vessel it extends in length to length L2 and then L3. As the front will usually be asymptotic, the length of the front can be discussed in terms of the length of the front between TH2 and TA2, these being within 3% of the peak temperature TH1 and start temperature TA1. If different criteria are set i.e. within 2% of the peak and start temperatures, then the nominated front lengths will be slightly longer.
For a certain store geometry a longer front will give lower thermal losses, but the length of the front will also reduce the useable amount of the store i.e. it will reduce the store utilization. If a store is 5 m in diameter and 10 m long and the thermal front is 5 m of this length, then the store utilization is reduced to approximately 50%.
If the same sized store was used and the particle size was reduced, then the same level of thermal losses could be achieved with a much shorter front. So a smaller particle size in a packed bed or pore size in a porous media will tend to give better heat transfer, lower thermal losses and better store utilization (a shorter thermal front). The one disadvantage is that there is a pressure drop associated with the fluid flow through the bed and this pressure drop increases significantly as the particle or pore size reduces.
Pressure is not a vector quantity, but a pressure gradient may be defined with respect to distance. The resistance to fluid flow increases with a decrease in the particle size and gives rise to a pressure drop in the fluid (δP). In the case of a thermal store there is a certain pressure drop δP over a store of length L, which in this case means the pressure gradient is δP/L. The pressure decreases in the direction of the fluid velocity so the gas pressure will be lower after the gas has passed through the store. This pressure drop is also the reason why the particle size in packed beds is not reduced to a very small size that will give much higher thermal reversibility. The losses from the pressure drop outweigh the benefits of the smaller particle size.
Another problem associated with the pressure drop over the store length is that, wherever possible, gas will tend to try to escape from the centre of the storage media and instead flow down the sides of the chamber bypassing the media leading to poor thermal exchange. It is, however, difficult to provide adequate sealing within a thermal store since they present particular issues of size and thermal cycling. As a store is charged or discharged, temperatures can vary by hundreds of degrees and as the thermal front progressed up or down the store the respective upstream and downstream sections experience relative thermal contraction/expansion which can lead to gaps of 2-20 cm for example in a large heat store. Allowance must therefore be made for such thermal expansion effects, while retaining adequate sealing.
Applicant's earlier application WO2011/104556 describes a thermal store in which the size and type of media can be varied through the thermal store to either reduce the irreversibilities that are created when a thermal front is generated or else to reduce the pressure drop of a gas passing through the storage media by increasing particle size.
Thermal stores may be used in energy storage systems and, in particular, in Pumped Heat Electricity Storage (PHES) Systems, where at least one hot store and at least one cold store are required. Applicant's earlier application, WO2009/044139, describes a PHES system and that system 2 is illustrated schematically in FIG. 2. The system 2 comprises two large storage vessels 4,6 of particulate media 10 wherein electricity is used by a heat pump/engine machine 8 in a charging cycle (clockwise—as indicated by the arrow) to pump heat from one vessel 4 (the ‘cold’ store) to the other vessel 6 (the ‘hot’ store) resulting in the first vessel 4 cooling and the second vessel 6 heating. The electricity can then be regenerated by reversing the cycle (i.e. anti-clockwise direction) and passing the heat from the hot store 6 back through the machine to the cold store 4, while the machine 8 drives an electricity generator. The total energy storage is only limited by the size of the thermal energy stores, and hence, their design is critical to the overall system.
The present invention is directed towards providing a thermal energy storage apparatus of an improved design and, in particular, apparatus suited for use in an energy storage system.